In a large bowl, whisk together flour, sugar, and salt. Dissolve yeast in warm milk and let stand 5 minutes until foamy. Pour into flour mixture and stir until a shaggy dough forms. Knead on a lightly floured surface for 5 minutes until smooth. Wrap in plastic and refrigerate for 30 minutes.

Roll chilled dough into a 12x18-inch rectangle on a floured surface. Arrange cold butter slices over two-thirds of the dough, leaving a 1-inch border. Fold the unbuttered third over the middle, then fold the remaining third over top like a letter. Rotate 90 degrees, roll into a rectangle again, and repeat the letter fold. Wrap and refrigerate for 1 hour.

Make the hazelnut cream by combining ground hazelnuts, hazelnut paste, powdered sugar, heavy cream, and vanilla in a bowl. Mix until smooth and spreadable, adding 1 teaspoon more cream if too thick. Set aside at room temperature.

Roll the laminated dough into a 10x20-inch rectangle about 1/4-inch thick. Cut into 8 rectangles approximately 5 inches wide. Brush each rectangle lightly with egg wash.

Spoon 2 tablespoons of hazelnut cream onto the center of each rectangle. Fold the dough over the filling to create a pocket, pressing edges firmly to seal. Use a fork to crimp the sealed edges for extra security.

Place shaped pastries on a parchment-lined baking sheet, spacing them 3 inches apart. Cover loosely with plastic wrap and let rest in a warm spot for 45 minutes until slightly puffed.

Preheat oven to 375°F. Brush the tops and sides of each pastry generously with remaining egg wash, being careful not to let it pool on the pan.

Bake for 22-26 minutes, rotating the pan halfway through, until deep golden brown with crispy, shattered layers visible on the sides. The bottoms should sound hollow when tapped.

Transfer to a wire rack and cool for at least 15 minutes before serving. The filling will be molten hot immediately after baking and needs time to set to the perfect creamy consistency.